1. Field of the Invention
The present invention is directed to the field of liquid fuel tank monitoring systems and more specifically to the area of resistor cards employed in such systems.
2. Description of the Prior Art
In conventional liquid fuel level indicating systems for automotive vehicles such as shown in the 1987 Taurus/Sable shop manual published by the Ford Motor Co., Section 33-20, a variable resistor has its setting controlled by a float. The float is connected to a rod which is pivotally connected to sweep a wiper contact over a resistive card and thereby select a value of resistance indicative of the liquid fuel level within the tank.
A conventional resistor card 100, which is typical of those used in Ford Motor Co. vehicles, is shown in FIGS. 1A and 1B. That prior art resistor card 100 comprises a substrate 10 with a plurality of metallized conductors in groups 20, 22 and 24 depositied thereon. Each of the metallized layers is deposited on the substrate 10 so as to be electrically isolated, one from the other. A thick film resistive (ink) material 30, 32 and 34 is deposited in precise areas so as to interconnect respective groups of metallized layers 20, 22 and 24 to thereby provide precise resistant values between the metallized layers empirically determined by the shape and volume of the associated tank for which the fuel level is being measured. As can be seen in FIG. 1B, a wiper contact arm 40 makes direct contact at point 42 against the metallized layer. Although not shown, the wiper contact 40 traces an arcuate path corresponding to the arcuate arrangement of the metallized layers 20, 22 and 24 and makes contact with at least one of those layers throughout its path range.
When the card 100 is immersed within the liquid fuel, the fuel acts as a lubricant between the wiper 40 and the metallized contact layer 20, 22, or 24. However, when the card is dry because the level of the fuel in the tank has dropped below the location of the card 100, the wiper causes some wear to develop on the metallized layers. This problem has been noticed when vehicles are delivered cross-country from the factory, by rail or truck, where the fuel level in the tank is constant and the vibration caused by the transporting means produces fuel movement. Such movement causes corresponding and repeated movement of the wiper and significant wear on the metallized layers, with occasional wear-through to the substrate. In the event of wear-through, dead spots, (discontinuities) appear in the resistor card 100 and the fuel gauge observed by the operator is adversely affected when the wiper contact is in contact with such dead spots.